Call this to cause this FileDescriptor to lose its connection. It will
first write any data that it has buffered.

If there is data buffered yet to be written, this method will cause the
transport to lose its connection as soon as it's done flushing its write
buffer. If you have a producer registered, the connection won't be closed
until the producer is finished. Therefore, make sure you unregister your
producer when it's finished, or the connection will never close.